1. Field of the Invention
The present invention relates to a zoom lens and an imaging apparatus equipped with the zoom lens.
2. Description of the Related Art
In a zoom lens, particularly in a zoom lens for a single-lens reflex camera, a reflex mirror (swing-up mirror) is disposed in the camera body, hence a predetermined value or longer flange back distance is required to house the oscillating mirror and allow upward and downward movement without interference. This flange back distance must have a predetermined value or longer length, regardless of the focal length.
In the prior art, for the purpose of providing a predetermined value or longer flange back distance, a positive lens group is disposed in a lens group located in the rear part of the zoom lens. For current compact camera bodies, digital still cameras, etc., however, zoom lenses that do not require, and do not have, a long flange back distance have begun to be manufactured and sold.
On the other hand, various zoom lenses having a focus lens group decreased in size, or equipped with a camera shake correction lens, namely an anti-vibration which is required to correct camera shake, are now being manufactured and sold as specialized zoom lenses for video filming or the like.
Particularly, when auto focus is continuously performed at high-speed, such as in the case of video filming, wobbling is performed by movement within a part of the lens group (the focus lens group) at high-speed in the optical axis direction. For wobbling, the focusing state is changed, as in the sequence non-focused state→focused state→non-focused state, and the signal components in a specific frequency band of a specific portion of an image region are detected from the output signals of the image sensor during this change, whereby an optimum position of the focus lens group in the focused state is computed. By moving the focus lens group to this optimum position, the focusing operation is performed. In order to implement this wobbling efficiently at high-speed, it is extremely effective to decrease the size and weight of the focus lens group.
Since current camera bodies are being downsized and flange back distances shortened, a decrease in size of auto focus zoom lenses is now in strong demand. To downsize an auto focus zoom lens, it is effective to reduce the diameter of the focus lens group. Moreover, to increase the focusing speed, it is effective to make the focus lens group lighter in addition to reducing the diameter of the focus lens group.
In the case of a zoom lens having a camera shake correction lens as well, it is effective to make the camera shake correction lens group lighter in addition to reducing the diameter of the camera shake correction lens group, in order to decrease the driving load of the camera shake correction driving system, which is used for decreasing the influence of camera shake contributing to image deterioration.
Conventionally, a photoelectric conversion element (image sensor), which receives an optical image and converts it into an electric image signal, encloses an on-chip micro-lens or the like to receive incident light efficiently. This means that the inclination angle of the incident light to the photoelectric conversion element, with respect to the optical axis, is restricted. Conventionally, this restriction was satisfied by increasing the exit pupil diameter of the zoom lens to reach or exceed a predetermined value, so as to decrease the inclination angle of the incident light with respect to the optical axis, that is, securing telecentricity of the incident luminous flux that enters the image sensor. For example, one of the conventional methods for securing the telecentricity is to dispose a positive lens group in the rear part of the zoom lens.
In recent image sensors, however, aperture ratios have been improved, that is, the light receiving angle has been expanded, and the optical performance of the on-chip micro-lens has been improved, which lessens the restrictions on the position of the exit pupil and the size demanded for the zoom lens. For example, even if a negative lens group is disposed in the rear part of the zoom lens so that the luminous flux enters the optical axis of the image sensor obliquely, peripheral darkening (shading), due to the mismatch of the on-chip micro-lens and the exit pupil of the zoom lens, does not standout noticeably.
Moreover, recent years have witnessed advancement and improvements in software and camera systems. As a result, the problems generated by distortion can nowadays be eliminated or minimized by image signal processing, while with conventional technologies, extensive distortion causes major problems in image forming performance.